1. Field of the Invention
The present invention relates to ignition systems for internal combustion engine and, in particular, to an electronic ignition timing system for regulating ignition timing as a function of both engine speed and engine load.
2. Prior Art
The necessity for and advantages of regulating the ignition timing of an internal combustion engine are well known. A finite period of time is required for the complete combustion of the fuel/air mixture in each cylinder of the engine. Therefore, in order to realize full power from this combustion process under dynamic operating conditions, the fuel/air mixture should be ignited a predetermined time prior to the initiation of each piston's power stroke. The time that the fuel/air mixture should be ignited prior to the initiation of the power stroke is a function of the mixtures burn rate which is an inverse function of the quantity of the fuel/air mixture in each cylinder, turbulance and other factors. By defining the initiation of each piston's power stroke as the top dead center (TDC) position of the piston at the completion of the compression stroke, it is possible to determine when the fuel/air mixture should be ignited, with reference to the TDC position of the piston. In contemporary internal combustion engine systems, the ignition timing is mechanically controlled by means of a distributor timing shaft rotating in synchronism with the crankshaft of the engine. The control of the ignition timing is normally accomplished by a centrifugal advance and a vacuum advance mechanism. The centrifugal advance mechanism performs the primary function of causing the ignition to occur a predetermined time prior to the top dead center position of each cylinder as a function of engine speed. The vacuum advance system corrects the time of ignition as a function of the air pressure in the intake manifold which is indicative of the engine's load and provides a measure of the quantity of the fuel/air mixture in the cylinder. The operation of these pneumatic-mechanical ignition timing systems is well known and need not be discussed in detail.
The pneumatic-mechanical ignition timing systems are subject to wear and, therefore, less reliable than desired. Consequently, efforts are being undertaken to replace the pneumatic-mechanical ignition systems with electronic ignition timing systems. Various electronic ignition timing systems are known in the art which attempt to duplicate the ignition characteristics of the mechanical systems. Because electronic ignition timing systems cannot compute an ignition or spark advance from a reference signal indicative of the top dead center position of the associated cylinder, most electronic ignition systems use a reference signal which is advanced from the top dead center position, a fixed angular position which is greater than the maximum angular ignition advance required by the engine. The selected advance angle is generally greater than the angle through which the engine rotates, when operating at maximum speed during the predetermined time required for the full power combustion of the fuel/air mixture. The ignition advance time is then computed as a delay from the advanced reference signal.
Both analog and digital electronic ignition timing systems for internal combustion engines are taught by the prior art. Palozzetti et al, in U.S. Pat. No. 3,705,573, disclosed an analog ignition timing system which uses two sensors generating reference signals in advance of the top dead center position of the cylinder. The first sensor generates a reference signal which initiates the generation of a linear ramp voltage and the signal from the second sensor terminates the generation of the ramp signal. The value of the ramp voltage at the occurrence of the second signal is an inverse function of the angular advance required as a function of the engine speed. The generation of the ignition signal is then delayed from the occurrence of the second signal, a period of time inversely proportional to the speed of the engine as well as other engine parameters.
S. R. Finch, in U.S. Pat. No. 3,521,611 July 28, 1970, teaches a comparable type of electronic ignition timing circuit which uses two sensors to generate ignition signals in advance of top dead center as a function of engine torque and engine speed. As with the Palozzetti et al patent, Finch uses a first reference signal and a second reference signal, both angularly advanced of the top dead center position of the piston. The first reference signal initiates the discharge of a capacitor and the second reference signal terminates the discharge and initiates the recharging of the capacitor at a rate proportional to the speed of the engine. The delay in generation of the ignition signal from occurrence of the first reference signal is a direct function of the engine torque and an inverse function of the engine speed.
A third type of electronic ignition timing circuit is disclosed by G. Schirmer et al, U.S. Pat. No. 3,756,212 Sept. 4, 1973. The voltage of a saw tooth signal is compared to the signal generated by an adder circuit receiving input signals indicative of the engine speed, pressure in the engine's intake manifold and other engine parameters which may influence the timing of the ignition. The ignition signals are generated when the potential of the saw tooth signal is equal to the output of the adder circuit. In one embodiment of this circuit, the slope of the saw tooth signal is a function of the engine speed making the instantaneous voltage of the saw tooth signal a function of the angular position of the engine's crankshaft independent of engine speed.
Comparable digital electronic ignition timing circuits for internal combustion engines are taught by G. O. Huntzenger et al, U.S. Pat. No. 3,738,339, R. W. Asplund, U.S. Pat. No. 3,749,073 and H. Wakamatsui et al, U.S. Pat. No. 3,809,029.
The electronic ignition timing systems of the prior art treat the ignition advance as a linear function of engine speed, while it is known that the ignition timing advance, as a function of speed, is nonlinear due to turbulance within the cylinders as well as other factors. The disclosed electronic ignition timing system overcomes the problems of the prior art by treating the advance as a nonlinear function of engine speed and eliminates the need for multiple sensor to generate reference signals for operation of the system.